1. Field of Invention
The invention relates to a magnetic random access memory and, in particular, to a magnetic random access memory that has a lower switching field and power consumption in the ferromagnetic free layer.
2. Related Art
The magnetic random access memory (MRAM) is a type of nonvolatile memory. It utilizes the magnetoresistance property to record information and has the advantages of non-volatility, high density, high read/write speed, and anti-radiation. When writing data, a commonly used method is to use the intersection of the induced magnetic fields of two circuit lines, the bit line and the write word line, to select a cell. Its resistance is modified by changing the magnetization of a ferromagnetic free layer. When the MRAM reads recorded data, a current is supplied to the selected magnetic memory cell to read its resistance, thereby determining the corresponding digital value.
The magnetic memory cell between the bit line and the write word line is a stacked structure of a multi-layered metal material. It consists of a stack of a soft ferromagnetic layer, a tunnel barrier layer, a hard ferromagnetic layer, an antiferromagnet layer, and a nonmagnetic conductor. Controlling the magnetizations of the upper and lower layers of the tunnel barrier layer to be parallel or anti-parallel can determine the memory state to be “0” or “1”.
As the magnetic memory is designed to have a high density, the size of the memory cell should be decreased. This requires increase in the magnetic field for switching the magnetization of the ferromagnetic free layer, increasing the provided current. The large current makes the circuit design or the driver circuit design more difficult.
To solve the large current problem, most people adopt the means of changing magnetic memory cells so that their shape is closer to a circle. Although this method can reduce the switching field of the ferromagnetic free layer, the switching behavior of the magnetization of the ferromagnetic free layer becomes very complicated.
The U.S. Pat. No. 6,728,132 also discloses a solution. It primarily solves the discontinuous switching behavior of the magnetization vector of the ferromagnetic free layer. The ferromagnetic free layer is covered by a non-magnetic metal layer and a ferromagnetic layer. By adjusting the thickness of the metal layer, the magnetization vector of the ferromagnetic free layer and the covering ferromagnetic layer are anti-parallel to each other, forming closured magnetic field. However, it has limited effect to lowering the switching field of the ferromagnetic free layer.
As the capacity and density of memory both become larger, the write-in current needed by the magnetic memory also increases due to the structure of the magnetic memory cells. This imposes some difficulty in circuit designs. Therefore, it is necessary to provide a novel magnetic memory cell structure with a lower write-in current.